The invention relates to an electrochemical sensor for determining the oxygen content in gases, particularly in exhaust gases of internal combustion engines. A sensor of this general type is already known (DE-OS 37 37 215), and has a solid electrolytic body, at least one electrode on the side of the solid electrolytic body exposed to the measuring gas, as well as a protective layer with catalytically active substances on the electrode or electrodes exposed to the measuring gas, with the catalytically active substance being platinum or a platinum alloy. The incorporation of catalytically active substances into the porous protective layer which covers the electrode exposed to the measuring gas, is necessary in order to accelerate the setting of the thermodynamic equilibrium of the exhaust gas, and to achieve a control position as close as possible to .lambda.=1. However, it has been shown that at low exhaust gas temperatures, platinum is not capable of catalysing the equilibrium setting to a satisfactory degree. On the other hand, rhodium demonstrates an excellent catalytic effect at low temperatures, in particular with regard to the reduction of nitric oxides. In the publication SAE Paper no. 880557 "Multi-Layered Zirconia Oxygen Sensor with Modified Rhodium Catalyst Electrode", a lamina-shaped oxygen probe is proposed which has a rhodium layer on a platinum electrode, with an insulating intermediate layer being provided, where appropriate, between the platinum and rhodium layers. Through this layer-type construction, the equilibrium setting in the exhaust gas, and thus also the probe characteristics, are improved. The platinum electrodes and the rhodium catalyst are applied in layers one on top of the other, by means of screen printing. Thus at least two operating steps are necessary, and the consumption of precious metals is relatively high. If the platinum or rhodium layers are applied directly one on top 8 the other, without a separating insulating layer, then in addition an alloy formation results, whereby a portion of the rhodium loses its catalytic effect. The raw material, rhodium is, however, more than five times more expensive than platinum, and the amount used should therefore be kept as low as possible.